Microstructural development and electrical properties of sol-gel prepared lead zirconate-titanate thin films
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A systematic investigation of the microstructural evolution of fast fired, sol-gel derived Pb(Zr, Ti)O3 films (Zr/Ti = 54/46) was performed by analytical transmission electron microscopy (TEM). It was found that the nucleation and growth of the sol-gel PZT films were influenced by the precursor chemistry. The precursor solution was composed of Pb 2-ethylhexanoate, Ti isopropoxide, and Zr n-propoxide in n-propanol. Porous and spherulitic perovskite grains nucleated and grew from a pyrochlore matrix for NH4OH-modified films, but no chemical segregation was found. These thin films consisted completely of porous spherulitic PZT grains (^2 /j,m) when the firing temperature was increased. Chemical phase separation with regions of Zr-rich pyrochlore particles separated by Zr-deficient perovskite grains was observed in the initial stages of nucleation and growth for CH3COOH-modified PZT films. This phase separation is attributed to the effect of acetate ligands on the modification of molecular structure of the PZT precursor. Firing the acid-modified films at higher temperatures for long times resulted in porous perovskite grain structures. The residual porosity in these films is suggested to be a result of differential evaporation/condensation rates during the deposition process and the gas evolution at high temperatures due to trapped organics in the films. Dielectric and ferroelectric properties were correlated to the microstructure of the films. Lower dielectric constants (^500) and higher coercive fields (^65 kV/cm) were found for the acid-modified PZT films with phase separation in comparison to those measured from the sol-gel films with a uniform microstructure (s > 600, Ec < 50 kV/cm). All films fired at 650 °C showed relatively good remanent polarization on the order of 20 /zC/cm 2 .
I. INTRODUCTION Research in the field of ferroelectric thin films has recently gained great momentum owing to breakthroughs in the fabrication of thin layers of lead zirconate-titanate (PZT) materials.1 Applications of PZT ferroelectric thin films include nonvolatile memory cells, electro-optical devices, integrated optical modulators, and infrared sensors.1"3 Commercially viable ways of producing PZT films include RF magnetron sputtering from a ceramic target4 and sol-gel methods.5"12 Sol-gel technology offers advantages such as high chemical purity, good stoichiometric control, low-temperature processing, simple techniques (spin or dip coating) for thin film deposition, as well as microstructural control via chemical reactions.5 However, one drawback in the sol-gel processing of multicomponent oxide systems such as PZT is that a chemically inhomogeneous gel structure can result from
a 'Present
address: National Semiconductor Corporation, Santa Clara, California 95052-8090. b 'Present address: Department of Mechanical Engineering, Materials Division, University of California, Irvine, California 92717. 2208
J. Mater. Res., Vol. 6, No. 10, Oct 1991
http://journals.cambridge.org
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